Insulating phases of two-dimensional electrons in high Landau levels: Observation of sharp thresholds to conduction

The intriguing re-entrant integer quantized Hall states recently discovered in high Landau levels of high-mobility 2D electron systems are found to exhibit extremely nonlinear transport. At small currents these states reflect insulating behavior of the electrons in the uppermost Landau level. At larger currents, however, a discontinuous and hysteretic transition to a conducting state is observed. These phenomena, found only in very narrow magnetic field ranges, are suggestive of the depinning of a charge density wave state, but other explanations can also be constructed.